Now that Pluto is a dwarf planet, Mercury is the baby of the planets. A hollow container the size of Mars – the second smallest planet – would have room for almost three planets the size of Mercury.

Mercury probably used to be bigger than it is now.

Mercury could be described as a Moon-sized metal ball with a rocky crust, because its metal core is so big compared with the outer layers. This suggests that Mercury was once larger and that possibly a collision caused it to lose an outer layer.

You'd weigh about the same on Mercury as on Mars.

If you weigh 100 lbs on Earth, you'd weigh only 38 lbs on Mercury. But you'd weigh about the same on Mars even though it's about 40% bigger than Mercury. This is because Mercury has a high mass for its size, so its gravity is stronger.

Mercury is so close to the Sun that it's very hard to observe.

We know less about Mercury than many other planets. Astronomers must observe Mercury at twilight when it's low in the sky, and the features aren't clear. Half the planet had never been seen until 2008 when NASA's Messenger probe took pictures. Interestingly, 13 times a century we can see the disk of Mercury cross in front of the Sun. This is called a transit and the next one will be on May 9, 2016.

Astronomers used to think that Mercury did not turn on its axis.

Since Mercury is close to the Sun, we would expect it to keep the same side facing the Sun, as the Moon does while it orbits Earth. However in 1965, studying the planet with radar, astronomers discovered that it rotates three times in every two years. This still means the days are very long.

Mercury has the biggest temperature differences of any place in the Solar System.

The sunny side heats up to over 800 degrees Fahrenheit (430 degrees Celsius). That's hot enough to melt the metals tin, lead and zinc. On the dark side, the temperature can drop to -290 degrees Fahrenheit (-179 Celsius). This is as cold as some of the moons of Saturn, even though they are more than twelve times as far away from the Sun as Mercury is. By the way, the lowest temperature ever recorded at the surface of the Earth was a comparatively temperate -129 degrees Fahrenheit (-89 degrees Celsius).

Mercury has a very elliptical orbit.

The mean distance between Mercury and the Sun is 36 million miles. Yet it gets as far away as 43,380,000 miles (aphelion) and as near as 28,580,000 miles (perihelion). If you could stand on Mercury at perihelion, the Sun would look three times as big as we see it from Earth.

There may be water ice on Mercury.

Radar images show what seems to be ice at the north and south poles, deep inside craters that never get sunlight.

One of the biggest impact craters in the Solar System is on Mercury.

Mercury is cratered like the Moon. And something pretty big hit Mercury in its early days to make a hole the size of the Caloris Basin. This impact crater is about 960 miles (1550 km) in diameter. That's more than the distance from New York City to Chicago or from London to Rome.

Mercury is the fastest-moving Solar System planet.

Mercury's average speed is 67,000 miles per hour (107,000 km per hour). It's so close to the Sun, it has to move very fast to keep from being pulled in by the Sun's gravity. If you could go that fast, you could get all the way around the Earth at the equator in less than 25 seconds. No wonder Mercury has the name of the swift Roman messenger god.

A year on Mercury is 88 Earth days.

If you lived on Mercury you'd have a birthday about four times as often as you do on Earth. However I definitely wouldn't recommend the climate.

References:

(1) "The Discovery of Water Ice on Mercury" http://www.nrao.edu/pr/2000/vla20/background/mercuryice/
(2) Solar System Exploration: Mercury http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mercury